If you are using addcslashes() to encode text which is to later be decoded back to it's original form, you MUST specify the backslash (\) character in charlist!
Example:
<?php
$originaltext = 'This text does NOT contain \\n a new-line!';
$encoded = addcslashes($originaltext, '\\');
$decoded = stripcslashes($encoded);
//$decoded now contains a copy of $originaltext with perfect integrity
echo $decoded; //Display the sentence with it's literal \n intact
?>
If the '\\' was not specified in addcslashes(), any literal \n (or other C-style special character) sequences in $originaltext would pass through un-encoded, but then be decoded into control characters by stripcslashes() and the data would lose it's integrity through the encode-decode transaction.

natnospam

I have found the following to be much more appropriate code example:
<?php
$escaped = addcslashes($not_escaped, "\0..\37!@\@\177..\377");
?>
This will protect original, innocent backslashes from stripcslashes.

phpcoder

Forgot to add something:
The only time you would likely use addcslashes() without specifying the backslash (\) character in charlist is when you are VALIDATING (not encoding!) a data string.
(Validation ensures that all control characters and other unsafe characters are correctly encoded / escaped, but does not alter any pre-existing escape sequences.)
You can validate a data string multiple times without fear of "double encoding". A single decoding pass will return the original data, regardless of how many times it was validated.)

johannes

Be carefull with adding the \ to the list of encoded characters. When you add it at the last position it encodes all encoding slashes. I got a lot of \\\ by this mistake.
So always encode \ at first.